Third generation code division multiple access (CDMA) mobile systems, such as Universal Mobile Telecommunication System 3GPP (3rd Generation Partnership Program) Wideband CDMA include on the uplink channel pilot symbols which are used by the receiver for channel estimation and other demodulation functions.
FIG. 7 illustrates a block diagram of a conventional channel estimation receiver 700 that uses only pilot symbols to produce a channel estimate. For the lth finger, pilot symbols of the nth slot are extracted from the received descrambled and despread data channel (DPCCH) signal, and the functional section 701 computes a new slot average. This new slot average is filtered in the functional section 701 by a weighted mean square error (WMSA) non-causal channel estimation filter to produce a pilot-only based channel estimate ĉi,l. The estimates from the WMSA filter are further linearly interpolated at the control channel (DPCCH) symbol rate. The channel estimate is then input to complex conjugator 702 which calculates and outputs a complex conjugate of the pilot-only based channel estimate ĉ*i,l. Simultaneously, data on the data channel is delayed in delay 704 by D1 slots (to account for the non-causal WMSA filter delay) and channel-corrected, that is, multiplied in multiplier 703 by the complex conjugate of the pilot-only based channel estimate ĉ*i,l. While this pilot-only based channel estimation receiver 700 is simple, it has disadvantages in that there is poor channel estimate quality in adverse channels, thereby forcing the transmitter to increase either transmit power or coding gain at the expense of the information data rate.
FIG. 8 illustrates a block diagram of another channel estimation receiver 800 that builds on the receiver 700 of FIG. 7 in that it uses for channel estimation both pilot symbols and post-decoder output. Each of the fingers of the received signal is processed in fingers 1 through L, represented by reference numerals 801-1 through 801-L, respectively, to produce delayed, channel-corrected, data symbols for each finger. The data symbols of the fingers 801-1 to 801-L are then combined by combiner 802 to produce combined data symbol signals. This signal is input to post-receiver processing block 803, in which the signal is processed by deinterleaver/de-rate matcher 803-1, then a decoder 803-2, and then interleaver/rate matcher/re-modultor 803-3. The signal is then fed back to the fingers 1 through L over line 804. While this receiver 800 provides good channel estimates, it is prohibitively more expensive and complex in that it has a significantly increased latency and requires buffering of large chunks of data symbols. Such a receiver is described in M. Valenti and B. Woerner, “Iterative Channel Estimation and Decoding of Pilot Symbol Assisted Turbo Codes Over Flat-Fading Channels,” IEEE J. Sel. Areas in Commun., Vol. 19, No. 9, pp. 1697-1705, September 2001.
There is a need for an improved channel estimation receiver that strikes a balance between the simple but poor-quality channel estimation receiver 700 and the complex channel estimation receiver 800.